Transpiration in plants

Transpiration is the evaporation of water from plants. It occurs basically at the plant’s leaves while their stomata are open for the passage of carbon dioxide and oxygen during the chemical process of photosynthesis. The diagram of a plant below shows the movement of water from the soil into the leaves up to the leaves.

This transpired water must be replaced by the transport of more water from the soil to the leaves through the xylem vessel of the roots and stem.

1.      Importance of transpiration

Transpiration is the process that pulls water up from the roots to:

1)  supply water for photosynthesis,

2)   bring minerals from the roots for biosynthesis within the leaf, and

3)   Cool the leaf.

2. Translocation

The phloem conducts carbohydrates (mostly sucrose) to where they are needed by the plant as food. Food conduction in phloem is carried out through two kinds of elongated cells, the sieve cells and sieve tube members. The movement of sugars and other substances from one region to another through the sieve cells is called translocation:

3. Factors affecting transpiration

Investigating the factors affecting the rate of transpiration

Using a potometer (right), one can study the effect of various environmental factors on the rate of transpiration. As water is transpired or otherwise used by the plant, it is replaced from the reservoir on the right. This pushes the air bubble to the left providing a precise measure of the volume of water used.

4. Environmental factors that affect the rate of transpiration

(i) Light

Plants transpire more rapidly in the light than in the dark. This is largely because light affects the opening of the stomata. Stomata open fully when a plant receives more light than when it is dark.

(ii) Temperature

Green plants transpire faster at higher temperatures because water evaporates more rapidly as the temperature rises. For example, at 30°C, a leaf may transpire three times as fast as it does at 20°C. High temperatures make more water evaporate from leaves.

1. Humidity

The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increase. When the surrounding air is dry, diffusion of water out of the leaf goes on more quickly.

Wind

On windy days, transpiration increases as the wind blows around the leaves, blowing away any water vapour that diffused out from the leaf and could be on the surfaces When the air is still, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration.

Soil water

A plant cannot continue to transpire rapidly if its water loss is not by replaced from the soil. When absorption of water by the roots fail to keep up with the rate of transpiration, loss of turgor occurs, and the stomata close. This immediately reduces the rate of transpiration (as well as of photosynthesis). If the loss of turgor extends to the rest of the leaf and stem, the plant wilts.

5. Control of transpiration in plants

Plants in areas where rain is seasonal have specially developed features to overcome excessive loss of water by transpiration. We say they have adaptations for conserving water. The following are some ways of adaptations for plants:

(iii)  They shed their leaves in the dry season to survive through the dry season.

(iv)    Some plants have leaves whose surfaces are hairy or leaves with sunken stomata for reduction in transpiration.

(v)        Some trees such as mangoes, oranges, avocado and pine are evergreen. These retain their leaves all year round. The reduction in transpiration is because these leaves are covered with a layer of wax.

(vi)      Other plants such as pine and cactus have small and needle-like leaves to ‘allow very little transpiration.

(vii)   Fewer stomata on the upper surface of leaves like mango plants.

(viii) The upper surfaces are shinny, thus reflects some of the direct light.